J-poles and Slim Jims

What it is

An end-fed half-wave vertical with a built-in matching section, easy to build from copper pipe, wire or ladder line. Popular on 2 m and 70 cm for home, portable and repeater use, because it needs no radials and the matching is just geometry.

How it works

A half-wave element fed at its end has a very high impedance: high voltage, almost no current. Coax cannot drive that directly. The J-pole solves it with a quarter-wave shorted stub, a piece of transmission line shorted at one end. A quarter-wave line turns a short into an open, so impedance rises smoothly from near 0 Ω at the shorted bottom to very high at the open top, where the half-wave radiator takes over. Somewhere along the way it passes 50 Ω, and that is where the coax connects. See Stubs, quarter-wave sections and the gamma match.

Typical 2 m starting figures are 3.2 ft for the radiator and 1.6 ft for the stub (468 ÷ 146 and 234 ÷ 146). Real pole dimensions differ with the material, so trim the tap position and length for the lowest SWR.

J-pole versus Slim Jim

	J-pole	Slim Jim
Idea	Radiator over a shorted quarter-wave stub	The same matching idea, with the stub folded in beside the radiator
Built from	Pipe, wire or ladder line	Usually one piece of ladder line or twin-lead
Feed	Coax tapped on the stub	Coax tapped near the base

Either performs like a half-wave vertical: modest gain, vertical polarization, omnidirectional. Their appeal is simplicity and a good match with no radials, not gain.

In practice

• Common-mode current. The coax shield is tapped onto the stub, so RF can climb back down the outside of the cable. A few turns of coax as a choke below the tap, or a common-mode choke, keeps the feed line out of the pattern.
• Mount it clear. Keep it well away from metal masts, gutters and walls, which detune it and bend the pattern.
• Modest bandwidth. Trim the tap for the part of the band you use most.
• Portable. A rolled-up ladder-line version hangs from a tree branch or a camera tripod, which is why it appears in Portable and field antennas kits.

Related

• The quarter-wave verticalA monopole over ground: the workhorse omnidirectional antenna.
• 5/8-wave and colinear antennasSqueezing gain out of a vertical.
• Stubs, quarter-wave sections and the gamma matchMatching with pieces of transmission line.
• Common-mode chokesKeeping RF off the outside of the coax.
• Portable and field antennasLight, quick designs for SOTA, POTA and camping.
• Handheld radio antennasRubber ducks, whips and getting more from an HT.